Abstract
Historically important in the development of the Standard Model (SM) of particle physics, rare kaon decays are still a privileged tool for looking beyond it. The main reasons to continue the study ...
Highlights
The discovery of kaons and the study of their properties are seminal for the building of the Standard Model (SM)
We are still lacking a theory explaining why fundamental fermions appear in three families and many outstanding questions such as the baryon asymmetry of the Universe or the puzzle of dark matter remain unanswered
It is when the Higgs acquires a vacuum expectation value that the quarks get a physical mass and the charged-current W ± interations couple to the quark interaction eigenstates with couplings given by the Cabibbo-Kobayashi and Maskawa (CKM) matrix [2, 3]; (ii) So far all manifestations of CP violation and quark mixing are compatible with the single complex phase of the CKM matrix; (iii) Further experimental investment in the field of flavour physics is justified by the high energy scales addressed by these studies: in many cases they exceed the direct reach achievable at colliders
Summary
The discovery of kaons and the study of their properties are seminal for the building of the Standard Model (SM). The two decay modes K + → π+ννand KL0 → π0ννcan be used to quantitatively check the description of quark mixing and CP-violation independently from information extracted from the B system To see why this is the case it is convenient to express the formulas for the branching fractions in terms of contributions from the different loop functions. Together with the study of muon rare decays and searches for electric dipole moments of elementary particles, rare kaon decays like K + → π+ννand KL0 → π0ννoffer a genuine window of sensitivity to access high energy scales thanks to the absence of tree level contributions (CKM Unitarity), to the absence of long distance contributions (ννpair in the final state), and the hard GIM suppression at loop level in SM
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